This invention relates to multiple-color printers wherein imaging stations are disposed along an image receptor belt and, more particularly, to compensation for a disturbance of image registration induced by eccentricity of an idler roll disposed between two of the imaging stations.
One form of multiple-color printers employs a sequence of imaging stations disposed along a common span of an image receptor belt. By way of example, the belt may be carried by rolls wherein one of the rolls is a drive roll, a second of the rolls is a tension roll, and a third of the rolls is an idler roll. In the construction of such printers, it is important to maintain registration among the various color separations of the final image, which color separations are provided by the respective ones of the imaging stations. Construction of such printers is disclosed in United States Patents: Rees et al. U.S. Pat. Nos. 5,229,787; Dastin et al. 5,287,160; and de Jong et al. 5,287,162. In Dastin et al., placing all imaging stations at a distance from each other equal to an integer multiple of all component perimeters causes the cyclic error in velocity at each imager to be equal and in-phase, thereby eliminating the relative registration error. In the case of an image device such as a raster output scanner (ROS), the ROS synchronization and writing time might lead to errors in registration. These errors may be corrected by use of the mark-on-belt (MOB) sensor as taught by de Jong et al. Therein, the relative locations of different color chevron-shaped marks produced by each of the imagers is measured by the MOB sensor to enable corrective timing.
Practical rolls have significant eccentricity and cause the photoreceptor belt to have a cyclic error component in its velocity. The wavelength of this motion disturbance is too long to produce a perceptible single-color visual effect known as banding. However, unless the photoreceptor belt has the identical in-phase error at each imager, the various color separations do not overlap exactly. Such registration errors will cause shifts in hue, which may also appear in regular patterns known as rainbow banding. These effects are deleterious to image quality.
An aspect in the construction of the printers of the prior art is the locating of the successive imaging stations along a single span of the belt. Such a construction is useful in attaining a high degree of registration among the component images produced by the respective imaging stations. A disadvantage of such construction is the relatively large size of such a printer, the large size being due to the linear array of the in stations along the single span of the belt. In order to reduce the physical size of the printer, it is advantageous to locate the imaging stations on more than one span of the image receptor belt. With such a construction, an idler roll would be located between two of the imaging stations.
A problem arises with a construction wherein the idler roll is located between two of the imaging stations, this problem arising from a relatively small eccentricity within the idler roll. Such eccentricity may be characterized, in part, by a bump or portion of the idler roll with enlarged radius. As the bump engages with the belt, the speed of the belt upstream of the roll is increased momentarily while the consequent geometric alteration in the belt is taken up by the tension roll. As a result, the location of the image produced by an imaging station, located on the opposite side of the roll, is displaced slightly on the belt. In high-quality imaging, displacement of component images on the order of microns produces a noticeable effect on the final image. Therefore, there is a need to provide some form of compensation for the eccentricity of the idler roll.